1. Field of the Invention
The present invention relates generally to a read start pulse generator for an image signal processing system, and more specifically to a read start pulse generator incorporated in a time base corrector at a video tape recorder.
2. Description of the Prior Art
In an image signal processing system such as video tape recorder, a time base corrector is usually incorporated, by which a reproduced video signal is once written in a memory after A-D conversion and then read from the memory in synchronization with a stable reference pulse signal before D-A conversion.
The video signal the time base of which is thus corrected is outputted together with a synchronizing signal, a burst signal and a blank pulse signal all included in the reference pulse signal. In order to write or read the reproduced video signal in or from the memory, a write clock generator and a read clock generator are included in the time base corrector. The write clock generator generates a write start pulse signal to designate each head address of each scanning line in the memory and a write clock signal to sequentially increment the address in writing the video signal; the read clock generator generates a read start pulse signal to designate each head address of each scanning line in the memory and a read clock signal to sequentially increment the address in reading the video signal.
In the time base corrector shown in FIG. 1, it is necessary to allow the memory controller 5 to be able to automatically adjust the timing (i.e. the phase) of generating the write start pulse or the read start pulse in order to repeatedly write or read the image signal data for each scaning line in or from the memory under good reproducibility. Further, in writing or reading the video signal data in or from the memory, it is necessary to write or read the data corresponding to one cycle of color subcarrier signal as a unit in order to maintain the continuity of color subcarrier phase in the memory. That is, it is necessary to synchronize the timing of generating the write or read start pulse signal with that of the color subcarrier signal. In the case of NTSC (national television system comittee) type, since the frequency of the subcarrier is set to 455/2 times the line frequency, it is necessary to shift the phase of the write or read start pulse by a 1/2 period (about 140 ns) of the color subcarrier for each scanning line. In practice, in order to generate the write and read start pulses, an automatic phase controller (APC) is usually incorporated in the read clock generator and the write clock generator, respectively.
By the way, in a video tape recorder for broadcasting stations, there exists a need of a read clock generator by which the phase of the synchronizing signal or the burst signal can be adjusted by any given phase shift relative to the station's reference signal, where necessary, in order to take matching with other system connected to the time base corrector.
In the prior-art read clock generator of the time base corrector, therefore, since there are provided various adjusting devices such as horizontal synchronization phase adjusting device, a color subcarrier phase adjusting device, and a hue adjusting device, in addition to the horizontal synchronization color subcarrier phase adjusting device, if the above adjusting devices are operated after the horizontal synchronization color subcarrier phase adjusting device has been operated, there exists a problem in that the horizontal synchronization color subcarrier phase adjusting device should be adjusted again; that is, it is impossible to operate other adjusting devices, independently, thus causing a complicated time base corrector adjusting operation.
The arrangement and the operation of the prior-art read clock generator will be described sufficiently in detail hereinafter with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.